GaN, AlN, InN, which are nitride semiconductors, or materials made of mixed crystals thereof, have a wide band gap, and are used as high output electronic devices or short-wavelength light emitting devices. Among these, as high output devices, technologies are developed in relation to Field effect transistors (FET), more particularly, High Electron Mobility Transistors (HEMT). A HEMT using such a nitride semiconductor is capable of realizing high current, high voltage, and low on resistance operations, and is thus used for high output/high efficiency amplifiers and high power switching devices.
As a HEMT using a nitride semiconductor, there is disclosed a HEMT formed by sequentially forming a GaN layer and an AlGaN layer on a substrate made of, for example, sapphire, SiC (silicon carbide), GaN (gallium nitride), or Si (silicon), and using the GaN layer as an electron transit layer. The band gap of the GaN forming this HEMT is 3.4 eV, which is wider than 1.4 eV of GaAs. Therefore, a HEMT using a GaN layer as an electron transit layer is capable of operating in high breakdown voltage. Accordingly, considerations are being made to apply this HEMT to a high breakdown voltage power source. However, a HEMT formed with GaN usually becomes normally-on, and is thus unsuitable for applying to a power source. Thus, to make the HEMT become normally-off, for example, there is disclosed an HEMT in which a p-GaN layer is formed immediately below the gate electrode.
Patent document 1: Japanese Laid-Open Patent Publication No. 2002-359256
Based on FIG. 1, a description is given of a HEMT having a structure in which a p-GaN layer is formed immediately below the gate electrode. In a HEMT having this structure, a buffer layer 921, a GaN electron transit layer 922, and an AlGaN electron supply layer 923 are laminated on a substrate 910. On the AlGaN electron supply layer 923, a source electrode 932 and a drain electrode 933 are formed. According to the formed GaN electron transit layer 922 and AlGaN electron supply layer 923, 2DEG (two dimensional gas) 922a is formed in the GaN electron transit layer 922, near the interface between the GaN electron transit layer 922 and the AlGaN electron supply layer 923. Furthermore, in the area immediately below a gate electrode 931 on the AlGaN electron supply layer 923, there is formed a p-GaN layer 924, and the gate electrode 931 is formed on the p-GaN layer 924. Therefore, as the p-GaN layer 924 is formed immediately below the gate electrode 931, the 2DEG 922a disappears immediately below the p-GaN layer 924, i.e., immediately below the gate electrode 931, and therefore the HEMT becomes normally-off.
FIG. 2 illustrates results obtained by actually fabricating the HEMT having the structure illustrated in FIG. 1, and measuring the relationship between the gate voltage and the drain current. As illustrated in FIG. 2, in the HEMT having the structure illustrated in FIG. 1, when a gate voltage is applied, variations arise in the flowing drain current, and a hump is generated, so that there are cases where a drain current of approximately 1×10−6 A/mm through 1×10−7 A/mm flows even when a gate voltage is not applied. As described above, when a variation arises in the drain current in each element, a variation also arises in the on resistance. Therefore, when a HEMT having this structure is used as a power source, the properties of the power source become non-uniform. Furthermore, when a hump is generated, as described above, there are cases where a drain current flows even when a gate voltage is not applied. Therefore, the HEMT does not become completely normally-off, and there are cases where the HEMT breaks due to heat. Note that FIG. 2 indicates the properties of three HEMTs fabricated by the same process, and that that the applied drain voltage Vd is 1 V.